Suppress illegal reflective access in shared cache (#70355)

This commit temporarily supressess an illegal reflective access warning
by opening the java.io module to all unnamed modules. This is needed
when using the searchable snapshots shared cache due to some reflective
access that occurs there. This is temporary while we explore
alternatives.

distribution/tools/launchers/src/main/java/org/elasticsearch/tools/launchers/SystemJvmOptions.java

@@ -56,7 +56,8 @@ static List<String> systemJvmOptions() {
                 "-Dlog4j.shutdownHookEnabled=false",
                 "-Dlog4j2.disable.jmx=true",
 
-                javaLocaleProviders()
+                javaLocaleProviders(),
+                maybeAddOpensJavaIoToAllUnnamed()
             )
         ).stream().filter(e -> e.isEmpty() == false).collect(Collectors.toList());
     }
@@ -87,4 +88,19 @@ private static String javaLocaleProviders() {
         }
     }
 
+    private static String maybeAddOpensJavaIoToAllUnnamed() {
+        /*
+         * Temporarily suppress illegal reflective access in searchable snapshots shared cache preallocation; this is temporary while we
+         * explore alternatives. See org.elasticsearch.xpack.searchablesnapshots.preallocate.Preallocate.
+         *
+         * TODO: either modularlize Elasticsearch so that we can limit the opening of this module, or find an alternative
+         */
+        if (JavaVersion.majorVersion(JavaVersion.CURRENT) >= 9) {
+            return "--add-opens=java.base/java.io=ALL-UNNAMED";
+        } else {
+            return "";
+        }
+
+    }
+
 }